ISTANBUL — US Secretary of State Hillary Clinton on Thursday condemned the latest onslaught of violence sponsored by the regime in Syria and said President Bashar al Assad had to go.

“The regime-sponsored violence that we witnessed in Hama yesterday (Wednesday) is simply unconscionable,” she told reporters.

A solution to the Syria crisis required a cease-fire, a transfer of power and the formation of a representative interim government, she added.

“Assad must transfer power and depart Syria,” she said after a late-night strategy session with Arab and Western powers on how to increase the pressure on the Damascus regime and bring about political change.

Clinton acknowledged that the United States had not yet been successful in bringing about international action that would have an impact on Assad.

“We have to reiterate our unity, we have to send a clear message to other nations that are not yet working with us, or even actively supporting the Assad regime, that there is no future in that,” she said.

“And indeed planning for an orderly transition will be an important step.”

The intensifying diplomacy comes against the backdrop of fresh reports of massacres in Syria and growing fears in the region of a descent into a destabilizing civil war.

The views aired in Istanbul were expected to be taken up again Thursday at the United Nations when the Security Council meets to hear special envoy Kofi Annan’s report on his battered peace plan.

Clinton is sending her special representative on Syria, Fred Hoff, to Moscow on Thursday to sound out the Russians, the official said.

“She made clear that we want to work with Russia, but that we’ve got to have a common vision,” a senior State Department official said after the gathering with Arab and Western power.

On Wednesday, Russia and China rejected any moves to remove Assad from power in a joint statement after two days of meetings between Russian President Vladimir Putin and Chinese leaders.

Also today, UN monitors trying to get to the scene of a new massacre in Syria were shot at, UN leader Ban Ki-moon said, calling the latest atrocity “shocking and sickening.”

Ban told of the attack on the UN monitors in a speech to the UN General Assembly in New York when he said that Syria’s President Bashar al Assad had “lost all legitimacy.”

The observer mission said earlier Thursday it was being prevented from reaching the village of al Kubeir, site of the alleged killing of 55 people by pro-regime militias.

“We are receiving information from residents of the area that the safety of our observers is at risk if we enter the village of al Kubeir,” Maj. Gen. Robert Mood, the UN mission head, said in a statement.

“Despite these challenges, the observers are still working to get into the village to try to establish the facts on the ground,” he added.

Syrian state television denied that the monitors were being turned back by the Syrian army.

The United Nations was also due to hear from special envoy Kofi Annan on his battered peace plan for Syria.